UNIVERSITY OF CALIFORNIA, SAN DIEGO Understanding and improving the mechanical stability of semiconducting polymers for flexible and stretchable electronics A Dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in NanoEngineering by Adam David Printz Committee in charge: Professor Darren Lipomi, Chair Professor Joseph Wang, Co-Chair Professor Justin Opatkiewicz Professor Yu Qiao Professor Shyni Varghese 2015 Copyright Adam David Printz, 2015 All rights reserved. ii This Dissertation of Adam David Printz is approved, and it is acceptable in quality and form for publication on microfilm and electronically: ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Co-Chair ________________________________________________________________________ Chair University of California, San Diego 2015 iii DEDICATION To Catherine iv TABLE OF CONTENTS Signature Page . iii Dedication . iv Table of Contents . v List of Figures . x List of Tables. xiv Acknowledgements . xv Vita . xix Abstract of the Dissertation . xxii Chapter 1 Competition between deformability and charge transport in organic semiconducting polymers for flexible and stretchable electronics . 1 Abstract . 2 1.1 Introduction . 3 1.2 Device physics . 6 1.3 Deformability . 18 1.4 Inherent competition between deformability and charge transport . 25 1.5 Improving deformability without sacrificing electronic performance . 28 1.6 Conclusion and future directions . 33 Acknowledgements . 34 References . 34 Chapter 2 Best of both worlds: Conjugated polymers exhibiting good photovoltaic behavior and high tensile elasticity . 47 Abstract . 48 2.1 Introduction and background . 49 2.2 Experimental design . 54 2.2.1 Selection of materials . 54 2.2.2 Mechanical characterization . 54 2.2.3 Theoretical determination of tensile modulus . 56 2.2.4 Fabrication of devices . 57 2.2.5 Weakly interacting H-aggregate model . 57 2.3 Results and discussion . 59 2.3.1 Characterization of the polymers . 59 2.3.2 Mechanical properties of pure polymer films . 60 2.3.3 Mechanical properties of polymer:PC61BM composites . 65 2.3.4 Photovoltaic properties . 66 v 2.3.5 Microstructural characterization of the polymer films . 68 2.3.6 Correlations between tensile modulus and photovoltaic performance . 72 2.4 Conclusion . 74 2.5 Experimental methods . 75 2.5.1 Materials . 75 2.5.2 Synthesis of block and random copolymers . 76 2.5.3 Preparation of substrates . 78 2.5.4 Preparation of polymer solutions . 78 2.5.5 Fabrication of solar cells . 79 2.5.6 Characterization of materials . 80 Acknowledgements . 80 References . 81 Chapter 3 Viability of stretchable poly(3-heptylthiophene) (P3HpT) for organic solar cells and field-effect transistors . 89 Abstract . 90 3.1 Introduction . 90 3.2 Results and discussion . 92 3.2.1 Characterization of the polymers . 92 3.2.2 Thermal properties . 93 3.2.3 Band structure . 95 3.2.4 Charge transport properties . 96 3.2.5 Combined mechanical and photovoltaic properties . 99 3.3 Conclusion . 103 Acknowledgements . 104 References . 104 Chapter 4 Increased elasticity of a low-bandgap conjugated copolymer by random segmentation for mechanically robust solar cells . 108 Abstract . 109 4.1 Introduction . 109 4.2 Experimental methods . 115 4.2.1 Materials . 115 4.2.2 General . 115 4.2.3 Synthesis of PDPP2FT-seg-2T . 116 4.2.4 Mechanical characterization . 117 4.2.5 Fabrication and testing of photovoltaic devices . 118 4.3 Results and discussion . 119 4.3.1 1H NMR . 119 4.3.2 UV-visible absorption . 120 4.3.3 Gel-permeation chromatography . 122 4.3.4 Tensile moduli of conjugated polymer thin films . 124 4.3.5 Photovoltaic characteristics . 126 4.3.6 Atomic force microscopy . 127 vi 4.3.7 Competition between photovoltaic performance and stiffness . 128 4.4 Conclusion . 130 Acknowledgements . 131 References . 132 Chapter 5 Role of molecular mixing on the stiffness of polymer:fullerene bulk heterojunction films . 137 Abstract . ..